1. Field of the Invention
This invention relates to the art of thermal dye transfer printing wherein a sublimable dye of a dye transfer sheet is transferred to a dye receiving sheet in an imagewise pattern and more particularly, to a thermal dye transfer method and a thermal printing system of the type which comprises dye transfer sheets using sublimable dyes and dye receiving sheets in combination and which is adapted for multiple copying or printing wherein the dye transfer sheet is repeatedly used while the dye receiving sheet is in single use or the transfer sheet is run at a speed of 1/n, where n&gt;1, of the dye receiving sheet. The invention also relates to the dye transfer sheet used in the above system, and to a method for making the dye transfer sheet.
2. Description of the Prior Art
The thermal dye transfer printing systems using sublimable dyes are full color hard copy printing systems where printing with a density graduation for each printing dot is possible. However, dye transfer sheets having a sublimable dye layer are expensive. Many attempts have been made to utilize the dye transfer sheet repeatedly several to ten and several tens times for reduction of the costs. This multiple-use multi-usable mode printing where the dye transfer sheet is repeatedly used several to ten and several times has been reported, for example, in (1) "Partially Reusable Printing Characteristics of Dye Transfer-type Thermal Printing Sheets" in Proceedings of 2nd Non-impact Printing Technologies Symposium, pp. 101 to 104 (1985) and (2) "&lt;ulti-usable Sublimation Dye Sheets" in National Convention Record of the Institute of Image Electronics Engineers (June, 1986). The above reports (1) and (2) deal with the printing characteristics in the multiple-use mode determined by a relative speed system where the dye transfer sheet is moved at a speed, relative to a thermal printing head, smaller than the dye receiving sheet. Broadly, the multiple-use mode printing includes a simple repetition technique where one dye transfer sheet is repeatedly applied at n times where n&gt;1, and an n-times mode relative speed technique where the moving speed of the dye transfer sheet is 1/n of that of the dye receiving sheet, thus the dye transfer sheet being repeatedly utilized substantially n times. The relative speed system should permit the dye transfer sheet and the dye receiving sheet to move smoothly since these sheets are fed at different speeds. Since, however, a fresh portion of the dye transfer sheet is invariably supplied for the printing, the possible number of the repetitions in use of the dye transfer sheet becomes larger than that of the simple repetition system.
In the report (1), spherical spacer particles are provided between the dye transfer sheet and the dye receiving sheet, by which it is realized that at the repetition of n=12, a printing density is about 1.8. The report (2) states that the dye transfer sheet and the dye receiving sheet are in contact with each other to attain a printing density of about 1.0 at n=10.
In order to reproduce full color hard copies whose quality is similar to that of the hard copies obtained by ordinary single-use printing, it is required that the saturation printing density be substantially equal to that attained by the ordinary single-use printing and be in the range of about 1.5 to 1.8 and that the variation in printing density during the repetition of the printing operations be as small as possible when the same level of printing energy is applied for the respective operations so as to avoid an adverse influence as would be produced by the multiple-use mode printing.
With the above prior art (1), when the dye is provided in an amount sufficient for the multiple-use mode printing, the above requirements are satisfied with respect to the printing characteristics. However, it is necessary to provide a space between the dye transfer sheet and the dye receiving sheet. This imposes limitation on the type of dye which can be used in this system, i.e. the dye should have a high degree of sublimation. However, highly sublimable dyes generally involve the problem in practical applications that they are very poor in weatherability such as optical fading properties and fading in the dark. If a dye having a low degree of sublimation and thus high weatherability is applied to a system of the prior art (1), the printing density lowers considerably and thus, an intended printing density cannot be obtained. In the prior art (2), it is possible to use a dye of high weatherability and a low degree of sublimation since the dye transfer is effected under contacting conditions. However, the dye is used in an amount sufficient for multiple-use mode printing under which when printing is repeated at the same level of printing energy, the printing density greatly lowers as the number of repetitions increases. In addition, the printing density obtained by this mode printing does not reach a practically satisfactory level.